I (a native speaker from the Western U.S.) would say:
There is no such thing There is no such thing as a unicorn There is no such thing as unicorns There are no such things as unicorns
and *maybe*
There is not such a thing as a unicorn
but that feels pretty old-fashioned, somehow.
I would NOT say:
* There is no such a thing * There is no such a thing as a unicorn
Do you have links to the places where people wrote "no such a thing"? Maybe I'm forgetting some case, but I don't think I ever say or write that. Perhaps the writers originally wrote "no such thing", then changed it to "not such a thing" while editing, but only added the "a", forgetting to change "no" to "not".
There is no such a thing as...
I (a native speaker from the Western U.S.) would say:
There is no such thing
There is no such thing as a unicorn
There is no such thing as unicorns
There are no such things as unicorns
and *maybe*
There is not such a thing as a unicorn
but that feels pretty old-fashioned, somehow.
I would NOT say:
* There is no such a thing
* There is no such a thing as a unicorn
Do you have links to the places where people wrote "no such a thing"? Maybe I'm forgetting some case, but I don't think I ever say or write that. Perhaps the writers originally wrote "no such thing", then changed it to "not such a thing" while editing, but only added the "a", forgetting to change "no" to "not".